s390/idle: fix cpu idle time calculation
The idle time reported in /proc/stat sometimes incorrectly contains huge values on s390. This is caused by a bug in arch_cpu_idle_time(). The kernel tries to figure out when a different cpu entered idle by accessing its per-cpu data structure. There is an ordering problem: if the remote cpu has an idle_enter value which is not zero, and an idle_exit value which is zero, it is assumed it is idle since "now". The "now" timestamp however is taken before the idle_enter value is read. Which in turn means that "now" can be smaller than idle_enter of the remote cpu. Unconditionally subtracting idle_enter from "now" can thus lead to a negative value (aka large unsigned value). Fix this by moving the get_tod_clock() invocation out of the loop. While at it also make the code a bit more readable. A similar bug also exists for show_idle_time(). Fix this is as well. Cc: <stable@vger.kernel.org> Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com> Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
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@ -69,18 +69,26 @@ DEVICE_ATTR(idle_count, 0444, show_idle_count, NULL);
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static ssize_t show_idle_time(struct device *dev,
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static ssize_t show_idle_time(struct device *dev,
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struct device_attribute *attr, char *buf)
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struct device_attribute *attr, char *buf)
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{
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{
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unsigned long long now, idle_time, idle_enter, idle_exit, in_idle;
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struct s390_idle_data *idle = &per_cpu(s390_idle, dev->id);
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struct s390_idle_data *idle = &per_cpu(s390_idle, dev->id);
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unsigned long long now, idle_time, idle_enter, idle_exit;
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unsigned int seq;
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unsigned int seq;
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do {
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do {
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now = get_tod_clock();
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seq = read_seqcount_begin(&idle->seqcount);
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seq = read_seqcount_begin(&idle->seqcount);
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idle_time = READ_ONCE(idle->idle_time);
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idle_time = READ_ONCE(idle->idle_time);
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idle_enter = READ_ONCE(idle->clock_idle_enter);
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idle_enter = READ_ONCE(idle->clock_idle_enter);
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idle_exit = READ_ONCE(idle->clock_idle_exit);
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idle_exit = READ_ONCE(idle->clock_idle_exit);
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} while (read_seqcount_retry(&idle->seqcount, seq));
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} while (read_seqcount_retry(&idle->seqcount, seq));
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idle_time += idle_enter ? ((idle_exit ? : now) - idle_enter) : 0;
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in_idle = 0;
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now = get_tod_clock();
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if (idle_enter) {
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if (idle_exit) {
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in_idle = idle_exit - idle_enter;
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} else if (now > idle_enter) {
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in_idle = now - idle_enter;
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}
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}
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idle_time += in_idle;
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return sprintf(buf, "%llu\n", idle_time >> 12);
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return sprintf(buf, "%llu\n", idle_time >> 12);
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}
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}
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DEVICE_ATTR(idle_time_us, 0444, show_idle_time, NULL);
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DEVICE_ATTR(idle_time_us, 0444, show_idle_time, NULL);
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@ -88,17 +96,24 @@ DEVICE_ATTR(idle_time_us, 0444, show_idle_time, NULL);
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u64 arch_cpu_idle_time(int cpu)
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u64 arch_cpu_idle_time(int cpu)
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{
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{
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struct s390_idle_data *idle = &per_cpu(s390_idle, cpu);
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struct s390_idle_data *idle = &per_cpu(s390_idle, cpu);
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unsigned long long now, idle_enter, idle_exit;
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unsigned long long now, idle_enter, idle_exit, in_idle;
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unsigned int seq;
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unsigned int seq;
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do {
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do {
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now = get_tod_clock();
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seq = read_seqcount_begin(&idle->seqcount);
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seq = read_seqcount_begin(&idle->seqcount);
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idle_enter = READ_ONCE(idle->clock_idle_enter);
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idle_enter = READ_ONCE(idle->clock_idle_enter);
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idle_exit = READ_ONCE(idle->clock_idle_exit);
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idle_exit = READ_ONCE(idle->clock_idle_exit);
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} while (read_seqcount_retry(&idle->seqcount, seq));
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} while (read_seqcount_retry(&idle->seqcount, seq));
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in_idle = 0;
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return cputime_to_nsecs(idle_enter ? ((idle_exit ?: now) - idle_enter) : 0);
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now = get_tod_clock();
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if (idle_enter) {
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if (idle_exit) {
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in_idle = idle_exit - idle_enter;
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} else if (now > idle_enter) {
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in_idle = now - idle_enter;
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}
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}
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return cputime_to_nsecs(in_idle);
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}
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}
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void arch_cpu_idle_enter(void)
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void arch_cpu_idle_enter(void)
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